Cooling exhaust gases from smelting furnace

ABSTRACT

In combination with a furnace having a top from which very hot gases are exhausted, a cooling system has an outwardly open collar fixed to the top of the furnace and open inward into the furnace, a tubular exhaust stack adjacent the furnace, a connecting duct extending from the stack and fitted concentrically to the collar, a network of heat-exchange tubes lining the top of the furnace. The collar, and the connecting duct, and means for circulating through all of the tubes a coolant at generally the same temperature.

FIELD OF THE INVENTION

The present invention relates to a system for cooling exhaust gas. Moreparticularly this invention concerns the exhaust gases from a pig-ironreduction smelting furnace.

BACKGROUND OF THE INVENTION

In the production of pig iron, the furnace produces a great deal of hotexhaust gases that, on the one hand, contain valuable recoverable heat,and on the other hand should not be discharged directly into theatmosphere. Typically a tubular exhaust conduit or stack is attached tothe top of the furnace. A connecting conduit or duct extends from thestack to the furnace to conduct the hot gases from the furnace to thestack. As a rule both the top of the furnace and the stack are cooled bypassing a cooling fluid through them to a temperature Ti, as is theconnecting duct. The top of the furnace normally has a connection collarto which the connecting conduit is fitted.

The furnace of the device according to the invention generates exhaustgases at a superatmospheric pressure, typically about 0.8 bar aboveatmospheric pressure. The exhaust gas enters into the exhaust duct witha relatively high temperature of about 1450° C. In the gas-tight andcooled exhaust duct, the exhaust gases are cooled to a temperature thatis suitable for, e.g. preheating ore. The cooling of the exhaust or fluegases is done with a coolant that is conducted through cooling pipes inthe walls of the exhaust duct. The coolant or fluid is normally boilingwater at a temperature of for example 260° C.

Devices are known in which the inside wall of the furnace is cooled by acoolant pumped through cooling pipes lining the top of the furnace. Ascoolant, liquid water with a temperature of for example 60° C. isconducted through the cooling pipes of the inside wall of the furnace.Following cooling of the inside wall of the furnace, the heated liquidwater, which at this point has a temperature of 80° C. is disposed ofwithout utilizing the absorbed heat. These known devices have thedisadvantage that different thermal expansions, especially verticalthermal expansions of the various system components, result due to thetemperature differential between the cooling of the furnace inside wallon the one hand, and the exhaust duct on the other. This means thatelaborately equipped compensators need to be installed in order tocompensate for these thermal expansions. Furthermore, with many knowndevices, the cooling of the inside wall of the furnace, as well ascertain areas thereof is unsatisfactory.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved exhaust-gas cooling system.

Another object is the provision of such an improved exhaust-gas coolingsystem that overcomes the above-given disadvantages, in particular thatefficiently cools the gases with relatively simple but effectiveequipment.

SUMMARY OF THE INVENTION

In combination with a furnace having a top from which very hot gases areexhausted, a cooling system has according to the invention an outwardlyopen collar fixed to the top of the furnace and open inward into thefurnace, a tubular exhaust stack adjacent the furnace, a connecting ductextending from the stack and fitted concentrically to the collar, anetwork of heat-exchange tubes lining the top of the furnace. Thecollar, and the connecting duct, and means for circulating through allof the tubes a coolant at generally the same temperature.

Thus the invention is characterized in that the coolant for cooling thetop part of the furnace may be used for cooling both the inside wall ofthe furnace and the branch collars or sockets, and cooling pipes forcooling the upper part of the furnace extend to the collar of whichthere is at least one.

The temperature Ti of the coolant refers to the temperature at which thecoolant is fed to the exhaust stack or the connection duct and to thefurnace to be cooled. The coolant for cooling the top of the furnacelikewise has the same temperature Ti as the coolant for cooling theexhaust stack, which means that within the scope of the invention, thecoolant for cooling the furnace likewise essentially has the sametemperature Ti. The temperature of the coolant for cooling the top ofthe furnace may therefore vary up or down by as much as 15° C.,preferably 10° C., and even more preferably 5° C. from the temperatureTi of the coolant for the exhaust stack. According to an especiallypreferred embodiment, the temperature of the coolant for cooling the topof the furnace varies only up or down by 0° C. to 2° C. from thetemperature Ti of the coolant for the exhaust stack.

It is within the scope of the invention that the device according to theinvention preferably is operated with gas overpressure, i.e. that theexhaust gas enters the exhaust stack from the furnace atsuperatmospheric pressure so that the exhaust gas may have a pressure ofabout 0.8 bar above atmospheric pressure. Such gas overpressure isassociated with certain constraints or considerable mechanical stressesof the system components. Nevertheless, the device according to theinvention operates flawlessly, even at such a gas overpressure, whenimplementing the features according to the invention.

According to the invention, at least one collar, which is connectedbetween the top of the furnace and the exhaust stack , is likewisecooled with the coolant of the furnace. The connecting collar concernsan attachment branch for the end of the exhaust stack facing thefurnace. The coolant for cooling the furnace may therefore initially beused for cooling the inside wall of the furnace, and subsequently fedinto the connecting collar in order to cool the latter. The coolant withthe temperature Ti, however, may also be fed in parallel into theconnecting collar for cooling the latter and the top of the furnace inorder to cool the top.

For this purpose, the temperature Ti should exceed 150° C. , preferably200° C. , more preferably 230° C., and most preferably 240° C.

It is within the scope of the invention that the temperature Ti isbetween 240° C. and 280° C., preferably between 250° C. and 270° C. Thetemperature according to an especially preferred embodiment of theinvention is 260° C., or more or less 260° C. Thus both the coolant forcooling the inside wall of the furnace and the coolant for cooling theexhaust gas duct has this temperature Ti. The temperature Ti is thetemperature of the coolant fed to the inside wall of the furnace or theexhaust stack. Advantageously, the coolant for cooling the walls of thecollars has the temperature Ti.

It is within the scope of the invention that the cooling medium for thefurnace and the inside wall of the furnace and/or the connecting collarof the furnace is boiling water. Advantageously, both the top of thefurnace and the connecting collar are cooled with boiling water. Coolingwith boiling water ensures that a water/water vapor mixture developsfrom the boiling water, when cooled. Thus there is evaporative cooling.

It is within the scope of the invention that the exhaust stack and theconnection duct are also cooled with boiling water. The evaporativecooling for the furnace or the top of the furnace has very specialadvantages. The generated steam may therefore be used very efficientlyfor heat or energy recovery, as opposed to the devices known from therelated art. According to the invention the coolant for the furnace ispumped through cooling tubes or pipes that line the inside of the toppart of the furnace. Preferably, a cooling jacket is formed or coiledfrom these cooling pipes at the inside wall of the furnace or at theinside wall of the top part of the furnace.

According to the invention, the cooling pipes for cooling the furnaceand/or the top part of the furnace also extend into the collar, of whichthere is at least one. The cooling pipes are advantageously form coilsin the collar, and thus line the wall of the collar as a cooling jacket.

According to a preferred embodiment of the invention, the coolantinitially flows through the cooling pipes lining the inside wall of thefurnace, and subsequently into cooling pipes that line the wall of thecollar. This shared cooling for furnace and connecting collar has provento be successful. By using evaporative cooling for the device accordingto the invention it is possible to use relatively small cooling pipediameters, allowing the cooling pipes, as well, to be coiled withminimal bending radii relative to the cooling jacket. The diameters ofthe cooling pipes used for the furnace and/or connecting collar areadvantageously below 60 mm, and preferably in the 30-50 mm range. Due tothe very small bending radii of the cooling pipes, cooling jackets maybe realized allowing for very efficient cooling of the inside wall ofthe furnace and of the wall of the collars.

In an embodiment with two collars provided at the furnace, a narrowinside wall area of the furnace between these two collars may be cooledin a simple and operationally reliable way. This embodiment will beexplained in more detail below.

Evaporative cooling also has the advantage relative to the cooling ofthe inside wall of a furnace with removal of liquid water known from therelated art that corrosion and deposition problems in the cooling pipesmay be largely avoided.

Basically, for cooling the inside wall of the furnace and/or the collar,of which there is at least one, a continuous cooling jacket consistingof a coiled cooling pipe is used. According to an especially preferredembodiment of the invention, the cooling pipes, however, form a coolingjacket consisting of a plurality of cooling jacket sections throughwhich flows may pass separately and preferably parallel, and that may beblocked individually, if required. The individual turns or pipes of thejacket directly abut one another so as to completely cover or line theinterior of the space they are in. Hence, the cooling pipes according tothis embodiment form separate cooling coils. An especial advantage ofthis embodiment is that defective cooling jacket parts may be turned offwithout compromising the cooling effect of the other cooling jacketsections. In case of a leak in a cooling pipe, the whole device needstherefore not be shut down; instead a defective cooling jacket sectionmay be disconnected and exchanged, provided the other cooling jacketsections continue to operate.

According to an embodiment of the invention already mentioned above, twoadjacent collars are provided at the top of the furnace, and the anglebetween them does not exceed 100°, advantageously 95°, preferably 90°,and very preferably 85°. According to an especially preferred embodimentof the invention, the angle between the adjacent collars is about 80°.The angle is thereby measured between the center lines or axes of thenormally cylindrical collars. For an embodiment of the device accordingto the invention as space-saving as possible and of little volume, aminimum angle between the collars is chosen. This will result in a verynarrow area of the inside wall of the furnace between both collars,where cooling is problematic with devices known from the related art. Itis highly recommendable that this narrow space also be cooled so as toavoid any problems.

According to a very preferred embodiment of the device according to theinvention, cooling pipes are guided out of the first connecting collarover the narrow inside wall section of the furnace between bothattachment or intake holes of the collars, the cooling pipes then passinto the second connecting collar to cool it. This will allow simple andefficient cooling of the narrow space. This is especially the case,since within the scope of the invention cooling involves evaporativecooling, which makes possible small pipe diameters and especially smallbending radii of the cooling pipes. Hence, trouble-free cooling,including of the narrow space, may be achieved with the device accordingto the invention.

According to a very preferred embodiment of the invention, cooling pipesextending along the inside wall in order to cool the furnace pass intothe first connecting collar to cool the latter, and then again pass outof the first collar. Subsequently, these cooling pipes are pass over theinside wall section of the furnace between both attachment holes of thecollars, and then pass into the second connecting collar to cool thelatter, and then again pass out of the second collar. Then, thesecooling pipes may extend further along the inside wall of the furnace.As already emphasized above, evaporative cooling with boiling waterenables small bending radii, allowing trouble-free coiling, as wellfitting of the cooling pipes to the system sections.

The invention is based on the recognition that unwanted relative thermalexpansion, especially vertical thermal expansion between the furnace andexhaust stack may be avoided in an efficient and operationally reliableway by cooling the furnace with a cooling medium at the same temperatureas the exhaust stack. In contrast to the devices known from the relatedart, it is possible to omit elaborate compensators in the transitionarea between the furnace and exhaust stack. The invention is furthermorebased on the knowledge that especially efficient cooling of the furnaceand thus also effective avoidance of disturbing thermal expansion may beachieved if evaporative cooling is used during cooling of the furnace.Moreover, cooling with boiling water advantageously enables efficientheat recovery, which is not easily done with liquid cooling water usedin accordance with the prior art. When using evaporative cooling for thefurnace, signs of corrosion and deposits in the cooling pipes on thewalls of the furnace may furthermore largely be avoided.

The invention is also based on the recognition that by using the coolingmeasures according the invention, i.e. evaporative cooling, cooling maybe done relatively smoothly including at the connecting collar attachedto the furnace, and in areas that are not easily accessible. This isprimarily due to the fact that evaporative cooling makes possible theuse of cooling pipes with a small diameter, and thus also allows forsmall bending radii of the coiled cooling pipes. A further advantage ofthe invention is the possible avoidance of adverse condensation ofsulfur dioxide on the cooled inside walls of the furnace, when coolingsame at a relatively high temperature Ti (e.g., 260° C.). Finally, itshould be emphasized that the device according to the invention isdesigned in a relatively simple and uncomplicated way, making itrelatively inexpensive to manufacture.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a partly schematic view of the furnace and cooled exhaust-gassystem according to the invention;

FIG. 2 is a large-scale horizontal section through a detail of thesystem;

FIG. 3 is a large-scale view of a detail if FIG. 2;

FIG. 4 is a top view. of a double-collar furnace top in accordance withthe invention; and

FIG. 5 is a developed view from inside of the structure of FIG. 4.

SPECIFIC DESCRIPTION

As seen in FIG. 1 a melt reduction furnace 1 for the production of pigiron is associated with a tubular exhaust-gas stack 3 for removal andcooling of the exhaust gases is attached at the top part 2 of thefurnace. The exhaust gas enters the exhaust stack 3 with a temperatureof about 1450° C. and under a gas overpressure of about 0.8 bar aboveatmospheric pressure.

A connecting duct 4 attached to the furnace 1 is cooled by means of acooling medium having a temperature Ti and coming from a supply 16. Inthe example, this cooling medium is boiling water and is fed to theexhaust stack 3 at a temperature Ti of 260° C. The cooling medium movesthrough cooling pipes 9 of the exhaust stack 3. The pipes 9 in theexample form the primary wall of the exhaust stack 3.

According to the invention, a top part 2 of the furnace 1 where theconnecting duct 4 is attached is also cooled with a cooling medium ofthe same temperature Ti as the cooling medium for cooling the exhauststack 3. In the example, the cooling medium for cooling the top part 2of the furnace 1 thus also has a temperature of 260° C. This temperatureTi refers to the cooling medium fed to the furnace 1. The cooling mediumin the top part 2 of the furnace 1 moves through cooling pipes 10 of thefurnace 1, and these cooling pipes 10 are provided at the inside wall ofthe furnace 1 of the top part 2 in the example. The cooling pipes 10 arehere coiled into a cooling jacket 11. According to a preferredembodiment of the invention, the cooling jacket 11 consists of aplurality of cooling jacket sections in a way that is not shown infurther detail, and these may be separately connected and disconnected.It is within the scope of the invention that boiling water is likewiseused for cooling the top part 2 of the furnace 1. Evaporative coolingfor the furnace 1 produces significant advantages, as already explainedmore extensively above. According to an embodiment of the invention,parts of the lower furnace section may also be cooled in the same way asthe top part of the furnace.

In the example according to FIGS. 1 to 3, the furnace 1 has at its toppart 2 a connecting collar 12 that is provided for attaching the exhauststack 3 or connecting duct 4. According to the invention this connectingcollar 12.is cooled with the cooling medium also used for cooling thetop part 2 of the furnace 1. In other words, the cooling medium for thefurnace 1 is conducted through cooling pipes 10 provided both at theinside wall of the furnace 1 and at the inside wall of the connectingcollar 12. Hence, the cooling medium for the connecting collar 12likewise involves preferably boiling water at the temperature Ti.

FIGS. 2 and 3 show the attachment of the exhaust stack 3 and theconnecting duct 4 to the connecting collar 12 of the furnace 1. Theconnecting duct 4 is fitted coaxially into the connecting collar 12 andfastened to the connecting collar 12. The cooling pipes 9 form theprimary wall of the connecting duct 4. The cooling pipes 10 form theprimary wall of the connecting collar 12. The cooling pipes 9 and 10both advantageously carry boiling water with a temperature Ti of 260° C.as the cooling medium. As already described above, relative thermalexpansion between furnace 1 or connecting collar 12 and exhaust stack 3may be effectively avoided in this way. Both the connecting duct 4 andthe connecting collar 12 have an outside insulation layer 13.

Preferably as shown in the example, the cooling pipes provided at theinside wall of the top part 2 of the furnace 1 extend from this insidewall into the connecting collar 12 and overlay the pipes 10 lining thefurnace top.1 and collar 12. The cooling pipes 10 are therefore coilednot just for the cooling jacket 11 in furnace 1, but also, as it were,into the connecting collar 12. In other words, the cooling pipes 10 forcooling the furnace 1 extend into connecting collar 12, as well. This isexplained in more detail below based on a special embodiment.

It can also be seen in FIG. 1 that the exhaust stack 3 is supported on afloor 17 by support elements or vertical supports 8. The verticalsupports 6 are preferably heated with a medium, as is the case in theexample, and this medium for the vertical supports 8 comes from thesupply 16 and has the same temperature Ti as the cooling medium for theexhaust stack 3 and as the cooling medium for the furnace 1 orconnecting collar 12. Hence, in the example, the medium for heating thevertical supports 8 also has the temperature Ti of 260° C. The mediumfor the vertical supports 8 likewise involves preferably boiling water.For this purpose, the vertical supports 8 are hollow, for example pipes,through which the medium flows. By heating the vertical supports 8,relative thermal expansion of the connecting ducts 4 and the posts 8 mayeffectively be avoided, so that no related stresses are produced. Thisentails the considerable advantage of eliminating the need forcompensators between the individual sections of the exhaust stack 3 totake up thermal expansion. The vertical supports 8 are supported at thebottom on one single load uptake surface 17. Since both the top part 2of the furnace 1 and the connecting collar 12, as well as the exhauststack 3 are cooled with a cooling medium of the same temperature Ti, theabove-mentioned load uptake surface for the supporting elements may bedefined in a very simple and accurate way. The supporting elements orthe vertical supports 8 according to the preferred embodiment and in theexample according to FIG. 1 are formed are pivoted at their upper andlower ends, that is each vertical support 8 is attached via a hingejoint to the exhaust stack 3, and preferably via a hinge joint to thefloor 17. This has the advantage that horizontal thermal expansions maylikewise be absorbed or compensated without any problems.

FIG. 1 furthermore shows that the exhaust stack 3 has theabove-mentioned horizontal or slightly inclined connecting duct 4 thatcontinues into a first vertical connecting duct or stack section 5 inwhich the exhaust gas is conducted upward. This first vertical exhauststack section 5 is connected to a downwardly U-shaped baffle section 6that in turn is connected to a second vertical stack section 7, in whichthe exhaust gas moves downward. In FIG. 1, the further treatment ofexhaust gas after the exhaust stack 3 is not shown. The cooled exhaustgas in the exhaust stack 3 may, for instance, serve to preheat ore forthe production of steel.

In the example according to FIGS. 4 and 5, two adjacent collars 12 and14 are provided at the top part 2 of furnace 1. In the example, theangle between these two collars 12, 14 as measured between the centerlines or axes of the collars 12 and 14 is 80°. This angle is chosen assmall as possible in order to make the system as compact as possible. Insimilar prior-art systems, this makes the very narrow space betweencollars 12 and 14 impossible to cool. The invention is based on theknowledge that efficient cooling of this narrow space or inside wallsection 15 of the furnace is essential for long-term and functionallyreliable operation of the device. According to the invention efficientcooling of the inside wall section 15 of the furnace between bothcollars 12 and 14 is made possible as shown in FIG. 5 in that thecooling pipes 10 that are coiled into a cooling jacket 11 of the furnace1 initially extend to the right side along the inside wall of furnace 1.These cooling pipes then pass as coils into the first connecting collar12, whereupon on the left side of the first connecting collar 12, thecooling pipes again pass out of this connecting collar 12, and then overthe narrow inside wall section 15 of the furnace. This ensures veryefficient cooling of the inside furnace wall section 15. The coolingpipes 10 on the left side of the inside wall section 15 of the furnacethen pass out as coils into the second connecting collar 14, and on theleft side of this second connecting collar 14, they again pass out ofthe connecting collar 14 and further along the inside wall of furnace 1.Guiding the cooling pipes 10 in this way is possible, since theinvention uses evaporative cooling, and because evaporative coolingallows cooling pipes 10 of a very small diameter, so that the coolingpipes 10 may also have very small bend radii, enabling them to be fittedto this complex shape between the two collars 12 and 14.

1. In combination with a furnace having a top from which very hot gasesare exhausted, a cooling system comprising: an outwardly open collarfixed to the top of the furnace and open inward into the furnace; atubular exhaust stack adjacent the furnace; a connecting duct extendingfrom the stack and fitted concentrically to the collar; a network ofheat-exchange tubes lining the top of the furnace, the collar, and theconnecting duct; and means for circulating through all of the tubes acoolant at generally the same temperature.
 2. The exhaust-gas coolingsystem defined in claim 1 wherein coolant is at a temperature above 150°C.
 3. The exhaust-gas cooling system defined in claim 2 wherein thecoolant is above 200° C.
 4. The exhaust-gas cooling system defined inclaim 1 wherein the coolant is boiling water.
 5. The exhaust-gas coolingsystem defined in claim 1 wherein the network of tubes comprises amultiplicity of tube sections that are individually controllable.
 6. Theexhaust-gas cooling system defined in claim 1 wherein the furnace tophas two of the collars extending at an angle of at most 100° to eachother.
 7. The exhaust-gas cooling system defined in claim 6 wherein thetwo collars extend at an angle of at most 85° to each other.
 8. Theexhaust-gas cooling system defined in claim 6 wherein the tubes liningone of the collars extend continuously into the furnace top and line thefurnace top between the collars and then extend into the other of thecollars.
 9. The exhaust-gas cooling system defined in claim 1 whereinthe tubes lining the connecting duct project into the collar and overlapthe tubes lining the collar.